Alistair Bunkall

Defence correspondent

Yemen is worse than a failed state; it is a country in complete chaos and it is impossible to see a realistic end to the vicious civil war that has created the world's worst humanitarian crisis and sliced the country four ways.

Sana'a, the capital, is controlled by Houthi rebels. This rebel group emerged from the northern mountains in 2014 to govern a city of around two million people.

Iran has sent arms and fighters to back the Houthis; Saudi Arabia in turn has thrown its support behind the internationally recognised government and carried out sustained bombing of Sana'a, drawing worldwide criticism because of the high number of civilian deaths.

Airstrikes on Sana'a seem to have lessened, perhaps in recognition that Riyadh's reputation was taking a battering too, but Saudi jets continue to target towns and villages in the north of the country, which goes largely undocumented.

Reports from the capital tell us there is a growing frustration among the population that the Houthis are unable to run an efficient system that works to the benefit of its citizens.

Cholera and diphtheria are rife, made worse by a Saudi imposed air and naval blockade.

Riyadh justifies this blockade as a tactic to prevent Iran smuggling arms to the Houthis, but the consequence is that vital aid - medicine, food and fuel - cannot get in, and so civilians are dying.

Popular discontent within Sana'a might be the best hope for the government forces battling to retake the capital because victory through military force looks to be a non-starter.

East of Sana'a, government forces have advanced to within 20 miles of the capital and now occupy the Nehm mountains.

Senior commanders told us with bravado that they are advancing day by day and will move on the capital "when the conditions are right", whatever that might mean.

From the evidence we saw, the government forces have neither the manpower, nor the equipment, to launch an assault on the capital which would almost certainly result in intense urban warfare.

Image:This 17-year-old boy lost his leg after joining up to fight in his school holidays

Until last week, the government also controlled the key port city of Aden in the south, which, incidentally, was a British colony until 1963.

Aden had become the seat of power, from where the country's prime minister (the president is living in exile in Riyadh) could control an arc of power running from top to bottom through the middle of the country.

But another group, the Southern Separatists, challenged that and have now seized control of the city.

To make matters more complicated, the Southern Separatists are backed by the UAE who, although part of a wider led coalition with Saudi Arabia, now find themselves at odds with Riyadh.

There is now momentum to carve Aden and the south off as a state of its own.

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Video:Civilian casualties mount in Yemen

And then there is al Qaeda in the Arab Peninsuar (AQAP).

The global terrorist organisation still controls much of the east of Yemen but is being heavily targeted by US drone strikes and special forces operations.

This is Yemen's war-within-a-war.

US and British intelligence agencies still regard AQAP as one of the chief terrorism threats to the West, and there are fears Islamic State fighters, chased out of Iraq and Syria, might go to join AQAP in Yemen.

It would be easy but remiss to dismiss the conflict in Yemen as irrelevant, just another civil war in the Middle East.

The humanitarian effects are devastating - up to 10,000 have been killed, 20 million have been displaced from their homes, eight million are on the brink of famine, and 75% of the population need aid.

It is an entirely man-made crisis.

Image:There are wars within the Yemen civil war, which means there is little prospect of any resolution soon

Strategically, Yemen is important too - not only are the region's two main powers, Saudi Arabia and Iran, using it to play out a proxy war, and not only must al Qaeda be stopped before it launches attacks in Europe, but the country also sits on the corner of a major shipping route.

The Bab-el-Mandeb Straits are just 18 miles across at their narrowest point.

Ships travelling from Europe to Asia via the Suez Canal must pass through here. It is a key transit point for world shipping.